99 ZX2 idle issues

[wzdoug]

I need some help verification on my 99 ZX2 idle problem. It revs clear up to 2800 rrpm when it first starts and is slow to come back down. It does the same thing when I put it in gear. When driving around 55 mph / 2000 rpm the rpms will drop by 500 rpm by moving the pedal slightly (I mean VERY slightly) and then return.

I unplugged the IAC and the idle retuned to normal, just a little lower. The CEL came on (I haven't taken it down for a code read yet). The high rpm at the start went away, as did the rpm drop while driving. Does this mean the IAC is faulty, or is it something else? I removed the IAC and cleaned it. I blew into the chambers, and neither sealed completely. Is this normal or is it leaking? I don't want to buy an expensive part like this unless I know its the problem. The MAF was recently replaced and I've cleaned the throttle body. Neither of these changed the symptoms.

[Intuit]

It revs clear up to 2800 rrpm when it first starts and is slow to come back down. It does the same thing when I put it in gear.

Normal behavior; especially during cooler weather.

When driving around 55 mph / 2000 rpm the rpms will drop by 500 rpm by moving the pedal slightly (I mean VERY slightly) and then return.

Could be the transmissions shifting? How's the mileage ? If it's really bad then the torque converter may not be engaging properly or there could be a valve-body issue developing??

I unplugged the IAC and the idle retuned to normal, just a little lower.

IAC is what allows it to deliver the necessary air to idle high during it's cold-start warm up phase. If you wait until it completely warms up and pull it, that may cause a stall. Pulling it could cause a CEL for that vehicle. Don't know...

[wzdoug]

Thanks for your input. The car runs fine with the IAC unplugged, the trans shifts ok while its unplugged. It only acts up when its plugged in. We replaced a speed sensor a few years back. The car has 105,000 miles on it. The trans had been rebuilt because there's a "rebuilt by" sticker on it. It was rebuilt before 60000 miles; that's when I got it.

Things point to the IAC, but I don't know where it gets its info.